Extensible Resource Identifier

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eXtensible Resource Identifier (abbreviated XRI) is a scheme and resolution protocol for abstract identifiers compatible with Uniform Resource Identifiers and Internationalized Resource Identifiers, developed by the XRI Technical Committee at OASIS. The goal of XRI is to provide a universal format for abstract, structured identifiers that are domain-, location-, application-, and transport-independent, so they can be shared across any number of domains, directories, and interaction protocols.

Contents

[edit] Background & Motivations

URIs have been very successful identifiers over the Internet. However the growth of the Web has led to new requirements for resource identifiers that are not easily met by standard URI syntax. One of these key requirements — internationalization — was ultimately met by the W3C and IETF by developing a new form of URI called an Internationalized Resource Identifier (IRI). The IRI specifications built on the URI standard by extending the character set to support the full range of Unicode characters.

With the growth of XML, Web services, and other ways of adapting the Web to automated, machine-to-machine communications, another set of requirements has emerged. These are the requirements to be able to identify a resource independent of a specific physical network path, location, or protocol because you need to:

  • Create structured identifiers with self-describing "tags" that can be understood across domains the same way XML documents provide a self-describing, domain-independent data format.
  • Maintain a persistent link to the resource regardless of whether its network location changes.
  • Delegate identifier management not just in the authority segment (the first segment following the "xxx://" scheme name) but anywhere in the identifier path.
  • Map identifiers used to identify a resource in one domain to other "synonyms" used to identify the same resource in the same domain, or in other domains.

By early 2003, these requirements led to the establishment of a new technical committee at OASIS whose goal was to create a new type of identifier that built on top of the IRI specification the same way the IRI specification built on top of the URI specification. The XRI TC was also charged with creating an optional resolution protocol based on HTTP and simple XML documents called Extensible Resource Descriptors (XRDs).

[edit] Features

  • URI- and IRI-compatibility — XRIs can be used wherever URIs or IRIs are called for.
  • Cross-references — An XRI can contain another XRI (or a URI), to any level of nesting. This enables the construction of structured, "tagged" identifiers that enable identifier sharing across domains the same way XML enables data sharing across domains.
  • Global context symbols — These are single-character symbols (=, @, +, $, or !) that provide a simple, human-friendly way to indicate the global context of an i-name or i-number. These are not required, but may be used within communities of interest that agree on their meaning and how they are resolved.
  • Peer-to-peer addressing — XRI syntax supports the ability for any two network nodes to assign each other XRIs and perform cross-resolution. That is, a top-level namespace authority can be referred to by names assigned by other parties. This aids in federating namespaces between organizations or communities of interest.
  • Decentralization — XRIs can be rooted in either centralized addressing systems (e.g., IP addresses or DNS domain names) or private/decentralized root authorities.
  • Delegation — Namespaces can be delegated to other namespace authorities.
  • Federation — Namespaces defined separately at any level can be joined together in a hierarchical or polyarchical fashion, and made visible and resolvable.
  • Persistence — The ability to express the intent that parts (or all) of an XRI are permanent identifiers that will never be reassigned.
  • Human-friendly and machine-friendly formats — XRI provides syntax both for identifiers that can be created and understood by humans easily, and those that are optimized for machine structuring/parsing.
  • Simple, extensible resolution — XRI offers a lightweight resolution scheme using HTTP and simple XML documents.
  • Trusted resolution — the XRI resolution protocol includes a trusted version that uses SAML assertions.
  • Multiple resolution options — XRI resolution can be independent of DNS.
  • Fully internationalizable, leveraging Unicode and IRI specifications.
  • Transport independent — XRIs are not bound to any specific transport protocols or mechanism.


As a specific example, say a library system uses URNs in the ISBN namespace to identify books and DNS subdomains to identify its library branches. HTTP URI syntax does not provide a standard way to express the URN for the book title in the context of the DNS name for the library branch. XRI cross-reference syntax solves this problem by allowing the library (and even automated programs running at the library) to programmatically construct the XRIs necessary to address any book at any branch. Examples:

 xri://broadview.library.example.com/(urn:isbn:0-395-36341-1)
 xri://shoreline.library.example.com/(urn:isbn:0-395-36341-1)
 xri://northgate.library.example.com/(urn:isbn:0-395-36341-1)

This ability to create structured, self-describing identifiers can be extended to many other uses. For example, say the library wanted to indicate the type of each book available. By establishing a simple XRI dictionary of book types, it can now programmatically construct XRIs that include this metadata,

 xri://broadview.library.example.com/(urn:isbn:0-395-36341-1)/(+hardcover)
 xri://broadview.library.example.com/(urn:isbn:0-395-36341-1)/(+softcover)
 xri://broadview.library.example.com/(urn:isbn:0-395-36341-1)/(+reference)

Note that there those who disagree that these are problems. For example, the W3C Technical Architecture Group is working on a draft document which attempts to refute some of the claims made by XRI proponents.

[edit] Applications

Examples of applications being developed using XRI infrastructure include:

[edit] Examples

(Note that none of these show the prefix "xri://", which is optional in XRIs when they are not in URI normal form, i.e, they have not undergone the specified transformation between XRI format and URI format.)

Example XRIs composed entirely of reassignable segments:

 =Mary.Jones
 @Jones.and.Company
 +phone.number
 +phone.number/(+area.code)
 =Mary.Jones/(+phone.number)
 @Jones.and.Company/(+phone.number)
 @Jones.and.Company/((+phone.number)/(+area.code))

Example XRIs composed entirely of persistent segments:

 !!1002!A7C5
 !!1002!A7C5/!D90F.88

Example of XRIs with mixes of persistent and reassignable segments (XRI allows any combination of the two):

 !!1002!A745/(+phone.number)
 @Jones.and.Company/!D90F.88/(+area.code)

[edit] Licensing

OASIS, which is the organization supervising the development this new standard, has been criticized by the Free Software Foundation for their patent policies [1]. The FSF claims that their patent policies may not allow conformance of software implementing them to the definition of Open Source. However, the committees responsible for developing the XRI and XDI standards have expressly declared their intention to make sure that the standards are unencumbered and completely Open Source compatible. The relevant text from their charter pages [2] and [3] reads:

In no event shall this Technical Committee finalize or approve any technical specification if it believes that the use, distribution, or implementation of such specification would necessarily require the unauthorized infringement of any third party rights known to the Technical Committee, and such third party has not agreed to provide necessary license rights on perpetual, royalty-free, non-discriminatory terms.

In addition, XDI.org, an international non-profit public trust organization that holds an exclusive license to the original patents underlying XRI and XDI technologies as well as their predecessor, XNS, has contributed these rights to the OASIS XRI and XDI technical committees.

[edit] See also

[edit] External links

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